The present invention concerns sheaths for tubular electrodes used in electrical accumulators or storage batteries. Such electrodes are used in alkaline, as well as in lead storage batteries and comprise a number of connected, electricity-conducting rod-like or wire-like bars which are surrounded by active material. The latter, in turn is surrounded by an electrochemically inactive sheath.
The most important application of tubular electrodes is in lead storage batteries but the invention is obviously not limited to these only.
The first sheaths for tubular electrodes consisted of tubes of hard rubber into which had been notched a large number of grooves or slits parallel to one another and at right angles to the tube's axis. Since their introduction, these tubes have been improved in various ways, and considerable progress has been made in the development of a tube consisting of two layers. These first consisted of an inner layer of braided glass fiber and a outer layer of a perforated plastic sheet or film. Several other types of sheaths were subsequently developed; among them types wih several tubes connected with one another. Also, individual sheaths consisting chiefly of braided glass fiber, which are impregnated in order of one layer have been developed, such as, for example, sheaths to obtain greater stiffness and protection against chemical action. Although this improved the strength of braided glass fibers, it resulted in decreasing the porosity of the sheath, thereby lowering its permeability to electrolyte.
Double-layered sheaths possess certain advantages, among them being a relatively simple manufacturing process. The great advantage from the storage battery technology viewpoint, is that electrodes with good mechanical qualities and a good length of service are obtained.
In the production of the electrodes one first casts a bar or rod of an electricity-conducting material, for example, a lead-base alloy. In this case the grid consists of a molding or border with a number of bars projecting from it and parallel to one another. Over these bars, the tubular sheaths are then drawn, whereupon the space between the sheath and the bars is filled with active material in the form of a powder. It is impossible to carry out this filling operation if the two materials of which the sheath is made are not tightly connected. The drawing of the tubes over the bars is also rendered difficult by a poor union of the two materials. It is, therefore, essential that the two different materials in a tubular sheath consisting of two layers should be securely connected with each other. In this way one obtains to some extent an improvement of the envelope's mechanical qualities; but this is particularly important in connection with the introduction of the active material.
Formerly, union of the materials in the two layers of the sheath was effected by gluing. This, however, resulted in several disadvantages. Depending on the type used, glue had to dry or harden while the two layers were held together by means of mechanical devices, entailing an increase in the manufacturing costs and unnecessarily large equipment for the production of the sheaths. As the periods required for hardening of the glue usually are relatively very long, solution-base glue has been used for the most part. This is not desirable, however, with regard to the working environment. The gluing process also makes it difficult to obtain a sufficiently good union of the two material layers without the addition of a considerable surplus of glue. Such a surplus is a great disadvantage, as it impairs the qualities of the electrodes which are produced with the sheaths. A surplus of glue may also bring about increased production costs and environmental problems. A disadvantage of greater importance in connection with gluing is that it may prove difficult to effect a satisfactory union between the two layers. This, of course, depends on which material is used, and the conditions can be influenced by a certain preliminary treatment of the material.